Gas-liquid contactors, also referred to as absorbers, are capable of removing substances such as gases and particulate matter from flue gases produced by power plants. Sulfur dioxide (SO2), for example, is produced by the combustion of fossil fuels. Sulfur dioxide emissions are of a particular concern because they are known to be hazardous to the environment. Wet flue gas desulfurization is a process in which sulfur dioxide is removed from flue gas in the most common process, by contact with an alkaline slurry using a spray tower.
Wet flue gas desulfurization typically uses calcium-based slurries, or sodium-based or ammonia based solutions to contact flue gases and remove sulfur dioxide therefrom. A slurry is a mixture of solids and liquid. Examples of calcium-based slurries include limestone (calcium carbonate; CaCO3) slurries and lime (calcium oxide; CaO). Such slurries react with acidic gases to form precipitates which can be collected for disposal or recycling. Contact between the alkaline slurry and acidic gases which are present in the flue gases, such as sulfur dioxide, result in the absorption of at least a portion of the acidic gases by the slurry.
After the desulfurization process, the slurry is typically accumulated in a tank, where the sulfur dioxide reacts with water to produce sulfites (SO3−2). The sulfites further react with oxygen coming from aeration air bubbled into the slurry to produce sulfates (SO3−4). This process may be referred to as forced oxidation. Disassociation of the lime or limestone within the slurry provides calcium ions which react with the sulfates to produce gypsum (CaSO4−2H2O). The gypsum can be resold and used, for example, in the home construction industry. Forced oxidation of the slurry received in the tank by aeration ensures that all of the sulfites will be reacted to form sulfates, and thereby maximize the production of gypsum.
A disadvantage of known aeration processes is that is difficult to control oxidation of the sulfite slurry received in the tank since it is difficult to measure or estimate the dissolved sulfite concentration. As a result, it is typical to provide more than a sufficient amount of air into the tank to ensure that generally all of the sulfite are oxidized. Providing the additional pressurized air can reduce the efficiency of the power plant because energy is required to generate the pressurized air. Another disadvantage with known systems is that providing excessive levels of air to the slurry oxidizes essentially all of the sulfites to form sulfates and any benefits arising from sulfites not oxidized is lost.